The present invention relates to robot controls and more particularly to pulse width modulators employable in digital robot controls to generate control signals for operating the power amplifiers and robot joint motors and thereby controlling the robot arm motion and positioning.
In the incorporated cross-referenced patent applications, there is disclosed a new completely digital robot control which provides advanced robot performance with high accuracy, high speed, high reliability and economy of manufacture.
The design of robot controls has been partly based on what has been learned from the design of numerical machine tool controls. In a typical machine tool control, a position control loop may be implemented in a microprocessor or other digital controller which generates an analog voltage representing a velocity setpoint for a power amplifier assembly. Servo control loops for velocity and torque are closed with analog circuitry located within the power amplifier assembly.
In more recent robot controls, both position and velocity control loops have been implemented in a microprocessor or other digital controller which generates a voltage output representing velocity error or current setpoint for the power amplifier assembly. Analog circuitry in the power amplifier closes the current loop.
Requirements placed on robot control systems differ significantly from those placed on numerical machine tool controls. The following are two basic differences:
1. Control loop parameters can typically be established to optimize control at the time of machine tool installation because time constants for the system are relatively constant for all operating conditions of the machine. Robot control parameters cannot be similarly fixed since robot time constants can vary significantly over the operating range of the robot arm due to large inertia changes caused by the manipulation of variable weight loads from fully collapsed to fully extended arm positions.
2. Machine tools typically required stiff accurate path control. Robot arms typically require dexterity which in turn is facilitated by control over the force applied by the robot arm to an object.
With a completely digital control configuration, special robot control needs can be more readily and more effectively met. In developing the digit robot control disclosed in the incorporated cross-referenced patent applications, it became necessary to conceive and employ a pulse width modulation (PWM) scheme which enables digital implementation of the current loop outside the power amplifier block, i.e., with (1) digital signals applied directly to the power amplifier to control directly switch condition time and (2) time stabilized motor current feedback sampling synchronized to the sampling frequency of the position and velocity loops.
The present invention is directed to such a PWM scheme and to a device for implementing the PWM scheme.